1. Field of the Invention
The present invention relates to a semiconductor memory device and, more particularly, to an SRAM (Static Random Access Memory) and a method for manufacturing the same.
2. Description of the Related Art
FIG. 27 shows an equivalent circuit diagram of an SRAM memory cell. A load transistor Q.sub.5 and a driver transistor Q.sub.3 form an inverter. A load transistor Q.sub.6 and a driver transistor Q.sub.4 form an inverter. The inverters are electrically connected to each other to form a flip-flop.
A transfer transistor Q.sub.2 connects an output cell node 1000 of the inverter that is formed by the load transistor Q.sub.6 and the driver transistor Q.sub.4 and a bit line (BL). A gate electrode of the transfer transistor Q.sub.2 is electrically connected to a word line.
Source regions of the load transistors Q.sub.5 and Q.sub.6 electrically connect to a power supply line V.sub.DD. Source regions of the driver transistors Q.sub.3 and Q.sub.4 electrically connect to a ground line V.sub.SS.
A transfer transistor Q.sub.1 connects an output cell node 1002 of the inverter that is formed by the load transistor Q.sub.5 and the driver transistor Q.sub.3 and a bit line (/BL). A gate electrode of the transfer transistor Q.sub.1 is electrically connected to a word line.
The flip-flop retains a state in which the cell node 1000 is at a voltage of 3V, for example, and the cell node 1002 is at a voltage of 0V, for example, as "1", for example. Also, the flip-flop retains a state in which the cell node 1000 is at a voltage of 0V, for example, and the cell node 1002 is at a voltage of 3V, for example, as "0", for example.
An SRAM may suffer a problem of an .alpha.-ray soft error. Materials for wiring layers, molding resin, and the like contain a very small amount of radioactive substances. The radioactive substances generate .alpha.-rays. The .alpha.-ray soft error is a phenomenon in which retained data is destroyed due to the .alpha.-ray. The destruction of retained data by the .alpha.-ray soft error will be described below in detail, with reference to the accompanying figure.
FIG. 25 is a cross-sectional view of a silicon substrate 200 in which the load transistor Q.sub.6 and the driver transistor Q.sub.4 are formed. The silicon substrate 200 is of a p-type. An n-well 202 and a p-well 204 are formed adjacent to each other. A source 212 and a drain 214 of the driver transistor Q.sub.4 are formed in the p-well 204. A p-type well contact region 216 is formed in the p-well 204. The well contact region 216 is isolated from the source 212 by a field oxide film 206. The well contact region 216 and the source 212 are electrically connected to the ground line V.sub.SS.
A source 218 and a drain 220 of the load transistor Q.sub.6 are formed in the n-well 202. An n-type well contact region 222 is formed in the n-well 202. The well contact region 222 is isolated from the source 218 by a field oxide film 210. The well contact region 222 and the source 218 are electrically connected to the power supply line V.sub.DD. The drain 220 is isolated from the drain 214 by a filed oxide film 208.
Next, the destruction of retained data by the .alpha.-ray soft error will be described with reference to FIGS. 25 and 26. As shown in FIG. 25, when the cell node 1000 is at 3V, for example, the drain 214 is at 3V, and the p-well 204 is biased to the ground line V.sub.SS. Therefore, because a diode formed by the drain 214 and the p-well 204 is inversely biased, a depletion layer is formed.
In this state, if an .alpha.-ray passes through the drain 214 and the p-well 204 and reaches the silicon substrate 200, the depletion layer of the diode is warped by the .alpha.-ray. As a result, electron-hole pairs are cut along the pass of the .alpha.-ray. As shown in FIG. 26, the holes flow into the well contact region 216 and into the ground line V.sub.SS. The electrons flow into the drain 214 that is at a high voltage. The flows of the holes and the electrons lower the drain voltage. As a result, the retained data is destroyed. In other words, in this example, the state of the cell node 1000 changes from 3V to 0V, and therefore the state "1" changes to "0".